Load-depth sensing of isotropic, linear viscoelastic materials using rigid axisymmetric indenters

TL;DR

This paper extends indentation analysis to viscoelastic materials using rigid axisymmetric indenters, deriving equations to determine material properties from load-depth data, and investigates the application of hereditary integral decomposition methods.

Contribution

It develops a mathematical framework for load-depth sensing of viscoelastic materials, extending elastic indentation analysis to include hereditary integral models.

Findings

Derived two key equations for viscoelastic indentation analysis.

Applied hereditary integral decomposition to experimental load-unload data.

Demonstrated feasibility of extracting material properties from sinusoidal indentation.

Abstract

An indentation experiment involves five variables: indenter shape, material behavior of the substrate, contact size, applied load and indentation depth. Only three variable are known afterwards, namely, indenter shape, plus load and depth as function of time. As the contact size is not measured and the determination of the material properties is the very aim of the test; two equations are needed to obtain a mathematically solvable system. For elastic materials, the contact size can always be eliminated once and for all in favor of the depth; a single relation between load, depth and material properties remains with the latter variable as unknown. For viscoelastic materials where hereditary integrals model the constitutive behavior, the relation between depth and contact size usually depends also on the (time-dependent) properties of the material. Solving the inverse problem, i.e., determining the material properties from the experimental data, therefore needs both equations. Extending Sneddon's analysis of the indentation problem for elastic materials to include viscoelastic materials, the two equations mentioned above are derived. To find the time dependence of the material properties the feasibility of Golden and Graham's method of decomposing hereditary integrals (J.M. Golden and G.A.C. Graham. Boundary value problems in linear viscoelasticity, Springer, 1988) is investigated and applied to a single load-unload process and to sinusoidally driven stationary state indentation processes.